1. Field of the Invention
The present invention relates to a velocity detector.
2. Related Art
In a motion control of a servo motor, an SIN signal and a COS signal are generated based on a movement distance of a machine or a movement angle of a shaft of the machine in a position detector. An arithmetic circuit of the position detector samples SIN signals and COS signals at intervals of a unit time, and calculates a moving velocity or an angular velocity from a distance difference or an angle difference between two samples and a time interval between two sampling operations.
For example, if coordinates obtained by the two sampling operations are (x1, y1) and (x2, y2) and a sampling time interval is t (sec), an angular velocity ω (rad/sec) can be expressed by the following Equation 0.ω=(θ2−θ1)/t=(tan−1(y2/x2)−tan−1(y1/x1))/t  (Eq. 0)In the Equation 1, θ1 (rad) and θ2 (rad) are absolute angles (declinations) at two sampling time points, respectively. As shown in the Equation 1, it is necessary to calculate the declinations from the coordinates so as to obtain the angular velocity ω.
To deal with the problem, use of a CPU or a custom LSI (Large-Scale Integrated Circuit) as the arithmetic circuit is proposed to realize the calculation of the Equation 1 using a digital logic circuit.
To calculate an arctangent (tan−1), it is necessary to do a division of sin/cos. However, two problems occur to calculation of the arctangent as follows. First, if a COS signal on a unit circle nears zero, an absolute value of a tangent (sin/cos) becomes quite greater. This results in a considerable increase in an error in the calculation result of the arctangent.
Second, it is necessary to do a division (sin/cos) so as to calculate the arctangent and the tangent. A divider is a circuit difficult to realize by a digital circuit. To realize the divider by a digital circuit, it is necessary to employ a digital circuit in far larger scale than an adder or a subtracter.
To realize a multiplier by a digital circuit, it is necessary to employ a digital circuit in far larger scale than the adder or the subtracter.
It is an object of the present invention to provide a velocity detection apparatus capable of highly accurately detecting a velocity of a moving body using a small-scale arithmetic circuit.